Create an index

Oracle

CREATE INDEX customer_city_idx ON customer(city) ONLINE;

ONLINE builds it without locking the table. Use CREATE UNIQUE INDEX for unique; (a, b) for composite.

CREATE INDEX CONCURRENTLY customer_city_idx ON customer(city);

CONCURRENTLY does not block writes to the table, but it cannot run inside a transaction block and is slower.

CREATE INDEX customer_city_idx ON customer(city);
-- Lock-free: ... ALGORITHM=INPLACE, LOCK=NONE;

InnoDB adds most indexes online; on large tables allow writes with LOCK=NONE.

CREATE NONCLUSTERED INDEX IX_customer_city
ON dbo.customer (city)
WITH (ONLINE = ON);

ONLINE is an Enterprise feature. For covering columns use INCLUDE (col1, col2).

db.customer.createIndex({ city: 1 })
// Unique: { city: 1 }, { unique: true }
// Built in the background without blocking (default behavior in 4.2+)

1 is ascending, -1 descending order. On large collections, building on a secondary first reduces load.

# Not applicable (core). If RediSearch is present:
# FT.CREATE idx ON HASH PREFIX 1 customer: SCHEMA city TAG

Secondary indexes come only with the RediSearch module; core Redis access is by key.